An exposure apparatus for manufacturing a semiconductor integrated circuit irradiates a wafer with an exposure beam of various wavelength bands in accordance with a circuit pattern. An exposure beam emitted by a light source illuminates a reticle via an illumination optical system. A fine circuit pattern formed on the reticle is imaged onto a wafer via a projection optical system by exposure and transfer.
In order to increase the integration degree of a semiconductor integrated circuit, the circuit pattern must shrink in feature size, which requires a higher resolving power of the projection optical system. Effective means for increasing the resolving power are a method of increasing the numerical aperture (also called NA) of the projection optical system and a method of decreasing the wavelength of an exposure beam.
As exposure beams, a g-line (wavelength: 436 [nm]), an i-line (wavelength: 365 [nm]), a KrF excimer laser (wavelength: 248 [nm]), and an ArF excimer laser (wavelength: 193 [nm]) have already been put into practice. An exposure technique by an F2 excimer laser (wavelength: 157 [nm]) as a next-generation exposure beam has been put into practical use.
The wavelength band of the F2 excimer laser is different from the conventional ones described above in that light is readily absorbed by oxygen or moisture. It is also known that light is absorbed by ammonia (NH3), carbon dioxide (CO2), organic gas, ozone, and the like. The atmosphere in the optical path of an F2 laser beam must be controlled to suppress a light absorber to low density. Hence, the atmosphere in the optical path around a reticle and wafer often exchanged on the exposure apparatus is also controlled similarly to the atmosphere in optical systems such as an illumination optical system and a projection optical system. As an effective atmospheric control method, the optical path space is purged with inert gas such as helium gas (He) or nitrogen gas (N2), or the optical path space is evacuated.
In terms of the throughput of the exposure apparatus, control of the optical path atmosphere enables exposure at high illuminance and shortens the exposure time, resulting in high throughput. To achieve higher throughput, however, the exchange time for reticles and wafers, which are frequently exchanged, must also be shortened. In recent years, a multiple exposure method of exposing the same shot on a wafer a plurality of number of times by using circuit patterns on a plurality of reticles has been put into practical use, improving the image quality of exposure/transfer on a wafer. Demands become stronger for shortening the time taken to transfer a substrate such as a reticle or wafer.
Prior art examples of a substrate transfer system for a reticle or the like in an exposure apparatus will be described.
Japanese Patent Laid-Open No. 7-321179 discloses a system of loading a reticle from a reticle library to a predetermined position in a temporary stocker by a multiaxial robot hand, and transferring the reticle to a reticle stage by a robot hand having vertical and rotating drive shafts.
Japanese Patent Laid-Open No. 2000-294496 discloses a substrate transfer system coping with an openable closed substrate vessel called an SMIF pod. In this system, a substrate in the SMIF pod is located into an exposure apparatus by an opening/closing elevating mechanism called an SMIF indexer. The substrate is transferred to a temporary substrate stocker by a first multiaxial robot, and transferred by a second multiaxial robot to a pre-alignment stage for aligning a substrate to a predetermined reference. The substrate is then transferred by a transfer hand to a substrate stage for holding a substrate.
Japanese Patent Laid-Open No. 6-260386 discloses an exposure apparatus and a reticle transfer system. This apparatus comprises a first chamber in which a reticle to be exposed is airtightly contained and the interior is purged with inert gas, a reticle exchange mechanism for exchanging the reticle with one to be exposed next, a second chamber in which the reticle to be exposed next is airtightly contained and the interior is purged with inert gas, and an opening/closing means for interrupting/allowing communication between the first and second chambers.
Of these prior art systems, Japanese Patent Laid-Open Nos. 7-321179 and 2000-294496 aim to shorten the substrate transfer time. Japanese Patent Laid-Open No. 6-260386 aims to enable reticle exchange without disturbing the inert gas atmosphere in the reticle stage space.
However, the prior art systems suffer from the following problems.
Japanese Patent Laid-Open Nos. 7-321179 and 2000-294496 do not disclose any atmosphere control means regarding the space around a reticle. These systems are not practical reticle transfer systems in an exposure apparatus using an F2 excimer laser as described above. If the atmosphere in the whole reticle transfer system disclosed is simply controlled to reduce a light absorber, the exposure apparatus becomes bulky, complicated in structure, and very high in cost.
Japanese Patent Laid-Open No. 6-260386 discloses a system of purging the space around a reticle with inert gas, as described above. However, the second chamber is a so-called load-lock chamber. In the layout of directly connecting the load-lock chamber to the first chamber, a reticle to be exposed next is supplied only after the completion of gas purge in the load-lock chamber for every reticle exchange. This system is effective only when the reticle exchange frequency is very low and the time required to complete gas purge of the load-lock chamber is shorter than the reticle exchange interval. If the reticle exchange frequency is very high, the throughput greatly decreases.